Molecular Simulation Prediction on SO₂ Gas Adsorption in Bipyridine Ligand-Based Square-Pillared MOFs

Increasing concentrations of toxic gases caused by the burning of fossil fuels necessitates the development of efficient porous materials for gas capture. Metal–organic frameworks (MOFs) have attracted a lot of attention as potential porous materials due to their effectiveness in adsorption of toxic gases. In particular, square-pillared metal–organic frameworks stand out for their exceptional potential toward gas adsorption, attributed to their remarkable surface area, thermal and chemical stabilities, and tunable properties. In this context, molecular simulations have been executed to observe and analyze the adsorption process of toxic flue gases such as SO2 and CO2 on MOFs. The present work deals with two different stable fluorinated MOFs named [Ni(4,4′-bipyridine)2(AlF5)]n (ALFFIVE-Ni-bipy) and [Ni(4,4′-bipyridine)2(NbOF5)]n, (NBOFFIVE-Ni-bipy) featuring AlF52– and NbOF52– anion pillars, respectively, comprising 4,4′-bipyridine as organic ligand and nickel as the central metal. The significance of utilizing the 4,4′-bipyridine ligands in these fluorinated MOFs enhances the SO2 gas adsorption and selectivity in the framework. Density functional theory has been implemented for geometry optimization, and Grand Canonical Monte Carlo simulations have been performed to forecast the adsorption isotherms. Both ALFFIVE-Ni-bipy (11.4 mmol/g) and NBOFFIVE-Ni-bipy (8.7 mmol/g) showed high SO2 adsorption capacity at 1 bar pressure, but ALFFIVE-Ni-bipy showed very good adsorption than other square-pillared MOFs and also unveiled good selectivity of SO2 gas. The coadsorption of binary SO2/CO2 and ternary SO2/CO2/N2 gas mixtures at ambient conditions indicated that the cost-effective aluminum (Al)-based square-pillared ALFFIVE-Ni-bipy is particularly suitable for acid gas adsorption.